Selective catalytic reduction (SCR) is a chemical process used for converting oxides of nitrogen (NOX) with the aid of a catalyst into diatomic nitrogen (N2) and water (H2O). In modern motor vehicles, SCR is frequently used as part of an exhaust emissions control system of an internal combustion engine to reduce the subject engine's release of post-combustion NOX into the atmosphere. Gasoline, as well as diesel, engine emissions may be treated with SCR.
In a vehicle equipped with SCR, a reductant, which may be an aqueous solution of urea, is typically injected into the engine's exhaust stream. An SCR equipped vehicle typically carries its reductant onboard in a specially designed reservoir. The reductant is dosed into the engine's exhaust stream during engine operation in proportion to consumption of the engine's fuel. Once in the exhaust stream, the reductant is absorbed onto the system's SCR catalyst where the conversion of NOX takes place.